The goal of UCSF Breast Cancer SPORE Project #4 is to develop and assess the translational potential of agents we have developed that force telomerase interference in breast cancer. This Project focuses on exploitation for clinical use of a new strategy: to turn the action of active telomerase against the breast cancer cells. In this current funding cycle, we have successfully demonstrated that a low threshold of expression of mutant-template telomerase RNA (MT-hTer) genes in human breast cancer cells is sufficient for a potent killing and growth inhibitory effect on these cells. The telomeres that result from MT-hTer action are toxic to cells, inducing a robust apoptotic response. Additionally, during the previous SPORE funding period, new science arising from the Blackburn laboratory's research on telomerase also led to two] unanticipated discoveries: first, that simply decreasing the endogenous telomerase level by ribozyme or RNA targeting methods rapidly decreased cancer potential. Specifically, we found that lowering overall telomerase diminishes the metastatic potential of cancer cells in vivo, and rapidly inhibited the growth of breast and other cancer cells in vitro. Second, cell death induced by MT-hTer expression is dominant and does not require the p53 or pRb checkpoint pathways. Based on these findings, we then showed that combining the expression of MT-hTer with small interfering RNA directed against the endogenous WT-hTER of cancer cells synergistically increases the potency of the MT-hTer effects in killing cancer cells. The following Specific Aims, which have the goal of bringing this work to the clinic, are to: #1 Further test and characterize the previously developed immunoliposome (ILS) constituted with Her2-targeting antibodydeveloped in SPORE Project 3containing the MT-hTer/anti-hTER siRNA construct (MT-Rx agent). In order to monitor MT-Rx efficacy we will use relevant biomarkers of response to the agent, suitable for early stage clinica trials. #2 Identify telomere/telomerase-based biomarker patterns predictive of apoptotic response t anticancer treatments and to specific MT-Rx therapy. We will identify the subset(s) of breast cancers that wil be most responsive to existing therapies and to MT-Rx using (i) a panel of 60 breast cancer cell line grouped by genomic and expression profiling, telomere maintenance status and other clinically relevan characteristics and (ii) patient-derived primary breast cancer cells, including stem/progenitor cell lines; tha targets the most sensitive patient subpopulation, as identified. #3. Validate and optimize the assays fo biomarkers of telomerase and telomere status on tumor and biopsy specimens, with the goal of validating] these assays per CLIA regulations in a CLIA certified laboratory such that the results can be used clinically.) Toward translation of MT-Rx, we will finalize the product configuration, perform initial manufacturing seal up, and evaluate initial toxicology targeted systemic delivery of MT-Rx agent in rodent models.